Heating apparatus



w. M. HALL HEATiNG APPARATUS Nov. 18, 1952 2 SHEETS-SHEET 1 Filed Jan. 11. 1947 km/zwra/r mum/*1 M. an

R W m m mw. 0mm m w M 2 w Mm E L m w 2 M J S m m P M m T m Nov. 18, 1952 Flled Jan 11, 1947 Patented Nov. 18, 1952 HEATING APPARATUS William M. Hall, Lexington, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass, a corporation of Delaware Application January 11, 1947, Serial No. 721,540

8 Claims.

This invention relates to heating apparatus,

and more particularly to a microwave heater including a microWave-energy-filled region of space which is of substantial size as compared to the wavelength of the microwave energy.

One object of this invention is to devise apparatus for improving the eificiency and uniformity of the distribution of microwave energy in a cooking device.

A second object is to devise apparatus for producing a substantially uniform integrated radiofrequency heat pattern in a cooking device.

Another object is to devise apparatus for cooking large volumes of food with the expenditure of a minimum amount of input power.

'A further object is to devise means for producing periodic changes in the field distribution in a radio-frequency cavity, whereby the integrated heating effect of the field is made substantially uniform.

A still'further object is to devise means for continuously changing the mode of the waves in a radio-frequency cavity, whereby the changing complex modes produce a substantially uniform overall heating effect therein.

An additional object is to provide a microwave cooking device in which food masses, whose linear dimensions are large compared to the wavelength of the microwave energy used, may be cooked in a substantially uniform manner.

Still another object is to accomplish the above objects in a simple yet effective manner.

The foregoing and other objects of the invention will be best understood from the following description of some exemplifications thereof, reference being had'to the accompanying drawings; wherein:

:FigJl is'a vertical sectional view through a microwave cooker embodying the invention;

Fig. 2 is a section taken along line 2-2 of Fig. l and looking in the direction of the arrows;

Fig. 3 is a view similar to Fig. l of a modified cooker;

Fig. 4 is a section taken along line 4- of Fig. 3;and.

Fig. 5 is an elevation looking at the rear wall of the oven of Figs. 3 and 4 from the inside thereof.

In microwave cooking devices, the problem arises of cooking foodstuifs having a rather large total area or volume. In order to accomplish this result, it is necessary to energize large regions of space with microwave energy, and for proper cooking this energization should be substantially uniform throughout the space. Also, it is desirable to accomplish such energization with a minimum expenditure of power.

In such a space the wavelength of the microwave energy is small in comparison to the linear dimensions of the oven, or in comparison to the linear dimensions of the food mass to be cooked or heated.

At any single frequency only one mode of oscillation can exist in such a space. In other words, there can be only one distribution of regions of high electric field intensity and regions of high magnetic intensity. Unless the walls are made highly absorbing so that substantially all incident radiation is absorbed, the differences in field intensity will be very great. The problem presented, therefore, is to minimize or eliminate the effect that such field distribution may have on the heating of the material to be cooked.

The electrical load which is presented to the source will, in general, change if this field distribution changes. This will occur if the frequency of the source is changed or if the quantity or nature of the food which is being heated is changed.

Referring now to the drawings, and more particularly to Figs. 1 and 2 thereof, a hollow rectangular prismoidal enclosure or cavity l is made of a suitable metal and has rather thin Walls as shown; enclosure l is adapted to serve as the oven of the heating or cooking apparatus. A container 2 of food may be placed inside oven i, said container resting on the bottom wall of the oven or being placed in any other suitable position therein while the food is being cooked. In order to allow access to the interior of the oven for placing the food therein and for removing the food therefrom, a rectangular opening 3 is provided in the front wall of the oven I, this opening being closable by means of a hinged metal door 4. When door 4 is closed, the enclosure is entirely closed, except for an opening [2 for the exciting means to be described hereinafter. Cavity I has an internal width w, an internal height h, and an internal depth d.

Numeral 5 generally designates an electrondischarge device of the magnetron type, which includes, for example, an evacuated envelope 6, made of highly conductive material, such as copper, and provided with a plurality of inwardly-directed, radially-disposed anode vanes l. The arrangement is such that each pair of adjacent anode vanes i forms, together with that portion of the envelope lying therebetween, a cavity resonator whose natural resonant fre quency is, as is well known to those skilled in the art, a function of the geometry of the physical elements making up the same. For the purposes of the present invention it is desirable that the dimensions of each such cavity resonator be such that the wave-length of the electrical oscillations adapted to be generated therein has a predetermined value, for example. of the order of ten centimeters. Wavelengths of this order lie in the microwave region of the frequency spectrum. However, this invention is equally applicable to radio-frequency energy of longer or shorter wavelengths within or without the microwave region.

Centrally located in envelope 6 is a highly electron-emissive cathode member 8, for example, of the well-known alkaline-earth metal oxide type, said cathode member being provided with conventional means (not shown) for raising the temperature thereof to a level sufiicient for thermionic emission.

The electron-discharge device 5 is completed by magnetic means (not shown) for establishing a magnetic field in a direction transversely of the electron path between the cathode and anode members thereof.

Magnetron 5 is energized from any suitable source (not shown) and when so energized delivers high-frequency electromagnetic energy having a predetermined wavelength to a coaxial transmission line 9, the inner conductor I of which is coupled to oscillator by a loop II and the outer conductor of which is connected to envelope 6. The inner conductor II! of line 9 extends directly into the interior of oven the number of different modes which will exist in a given range of frequencies can be made somewhat greater than it would be if the oven were entirely symmetrical. This will tend to make the operation of the'oven more uniform if the source is to operate at different frequencies. The dimensions w, h, and d of enclosure or cavity I are all large compared to the wavelength of the oscillations of oscillator 5, as stated above, and are all different. For example, to

may be 13 inches, 12 may be 14 inches, and h' may be 15 inches.

For any given freouency of operation, however, the unique distribution of field maxima and minima still exists. Making the oven irreg-- ular in shape does not change this condition and therefore some other means must be resorted to. Moreover, it has been found that great irregularities of the surfaces of the oven, for example, deep corrugations, tend to increase the difiiculty of providing uniform heating of food samples. For decreasing the difiiculty of providing uniform heating, it has been found desirable to introduce the radio-frequency more or less symmetrically into the oven, rather than from one corner, for example. therefore preferably centered with respect to the vertical side walls of the oven, as shown in Fig. 2, and is preferably somewhat above the horizontal midplane of the oven, as shown in Fig. 1.

Opening I2 is In order to provide uniform heating in the oven, it has been found satisfactory to move the food about in the oven so that all parts of the food are exposed to many different regions in the oven and the effects of the different field intensities are averaged out. It is within the scope of this invention to provide uniform heating of a body in the oven by moving said body with respect to the oven.

Another expedient which has been found satisfactory is to change the boundary conditions of the oven, thereby changing the field distribution. This can be accomplished without moving the side walls by introducing into the oven an additional reflecting or dielectric object which can then be moved about, causing a change in field distribution within the confined space.

One possible type of reflecting object is shown in Figs. 1 and 2. A thin metallic plate member M, which may have substantially square upper and lower faces, as shown in Fig. 2, is mounted horizontally inside cavity I near the upper end thereof. The dimensions of the faces of this plate member are somewhat smaller than the corresponding dimensions d and w of the oven, but the face dimensions of the plate are each a substantial proportion of the corresponding oven dimensions, such as on the order of three-fourths thereof, for example. Plate I 4 is fixed to a shaft I5 which extends through the upper wall of oven I by means of a hole in the central area thereof. Shaft I5 is mounted, for vertical reciprocation with respect to oven I, in a suitable bearing I5 fixed to the top wall of said oven.

Shaft I5 is adapted to be continuously reciprocated vertically, thereby continuously reciprocating or moving plate I4 between the upper and lower dotted-line positions shown in Fig. 1. In order to reciprocate shaft I5 vertically, one end of a link I! is pinned to the upper end of said shaft, the other end of said link being similarly pinned to the outer end of a crank arm I3 which is keyed to a shaft I9. A motor 28 rotates a shaft 2| through a speed reducing mechanism 22. A worm'23 is keyed to shaft 2I and drives a worm wheel 24 keyed to shaft I9 to rotate said latter shaft and vertically reciprocate shaft I5 and plate I4 at any desired relatively slow rate of reciprocation. This rate may be on the order of oscillations per minute, for example.

It will be apparent that, as metallic plate I 4 is vertically reciprocated, in effect the internal height h of cavity I is continuously and periodically varied to a predetermined extent, since metallic plate I 4 looks like the upper wall f enclosure I to the high-frequency electromagnetic waves inside the cavity.

7 Microwave energy is fed by coaxial line 9 and exciting probe or rod II! from magnetron oscillator 5 to the interior of oven I. It has been found that a large oven of the above-described type, containing such a moving plate-like member therein and excited by microwave energy from source 5 in the above-described manner. produces a very uniform overall or integrated radio-frequency heat pattern throughout the entire interior of enclosure I. It is not known exactly how or why this result is accomplished, but our present understanding of the theory of operation is substantially as follows.

Due to the relatively large dimensions of cavity I with respect to the wavelength of the radiofrequency energy, such energy supplied to the interior of the cavity is reflected back and forth in various directions by the inner walls thereof to establish a complex-mode radio-frequency field pattern in the cavity or enclosure. Because of the large dimensions of the cavity with respect to the wavelength of the energy supplied, a plurality of nodes and loops appear in the standingwave system set up within the cavity, producing a high-order mode or complex-mode pattern in said cavity, said pattern filling the entire volume of the enclosure in a more or less uniform manner. The cavity I, due to its substantial dimensions with respect to the wavelength, is a high-Q cavity, which means that there is a large amount of circulating (reactive) power within the cavity, setting up a high standing wave ratio in the oven which utilizes the expended power in a very eificient manner for heating purposes.

The effective dimension h is periodically varied by the movements of plate I 4, as described above. Due to these periodic changes of the effective internal dimensions of cavity I, the field distribution therein is changed periodicallybecause of the change in the distance which must be travelled by the Waves before reflection from plate member I4. The radio-frequency field pattern or field distribution in the cavity is therefore changed periodically; in other words, the mode present in the cavity is changed periodically with the periodic reciprocation of plate member I4, thus in effect stirring up the microwave energy in the cavity. Therefore the device may be termed a stirrer or mode changer."

Since the field distribution, or the mode pat tern, inside the cavity is periodically changing, and, since there is a complex mode pattern inside the cavity, the total or integrated heat provided in any one area of the cavity during a period of time will be substantially the same as that provided in any other equal area of the cavity during the same time, no matter where in the cavity the two comparable areas are located. Due to this heat uniformity, the oven is made very effective for cooking purposes.

It is also within the scope of this invention to change the field distribution in the cavity I .by frequency-modulating the output of magnetron 5, thus changing the wave mode in the cavity by effectively changing the dimensions of the cavity with respect to the wavelength of the osi cillations supplied thereto.

Now referring to Figs. 3-5, these figures show a modified reflecting-object type of mode changer or stirrer or field-distribution-changer, together with a modified feed line between the magnetron and the cavity. In Figs. 3-5, the same reference numerals are used for elements corresponding to those of Figs. 1-2.

'In this modification, magnetron 5 when energized delivers high-frequency electromagnetic waves to a hollow rectangular wave guide 42 through a coaxial transmission line 9 which is coupled to oscillator 5 by loop II. The central conductor II! of line 9 extends into the interior of waveguide 42 near one end thereof, through an opening provided in the center of the longer side of said guide, to serve as an exciting rod or exciting probe for said guide.

The end of guide 42 nearest this exciting rod is closed, while the opposite end of said guide is fastened to the rear wall of enclosure I by fastening means I3 and is open. The interior of guide 42 is placed in energy-transmitting relationship with the interior of the cavity I by means of a rectangular aperture I2 provided in the rear wall of said cavity, this aperture being of the same'size and configuration as the interior of guide 42 and being aligned with said guide to place the interior of said guide in communication with the interior of cavity l. Aperture I2 is located in a central position with respect to the vertical side walls of oven I, and is preferably located above the horizontal midplane of oven I.

If desired, the interior of wave guide 42 may be sealed off or masked from the interior of oven I by means of a plate of a material which is transparent to microwave energy but is not transparent to solids, liquids, or gases, such as Pyrex or quartz, for example, in accordance with the principles disclosed in the copending application of Norman R. Wild, Serial No. 709,- 987, filed November 15, 1946, now Patent No. 2,599,033, dated June 3, 1952. Such a plate could beplaced in a position covering and sealing the open (oven) end of guide 42 to keep undesirable food vapors and steam out of said guide, as explained in said Wild application.

A motor 20, through a, speed reducer 4|, drives a disk 25 by means of a friction wheel 26. Disk 25 is keyed to a shaft 21, which extends through a suitable bearing 28, in which said shaft is journaled for rotation, into the interior of oven I through an aperture provided centrally in the top wall of said oven. Shaft 21 is continuously rotated by motor 20 through the above-described mechanism.

Fastened to the lower end of shaft 21 is a metallic grid-like stirrer structure indicated generally by 29. A sleeve 30 is fastened to the lower end of shaft 21. An upper support rod 3| is integral with sleeve 30 and the axis of said rod extends radially from sleeve 30, so that said rod is horizontal. Four parallel rods 32, 33, 34 and 35, of smaller diameter than rod 3|,have their parallel axes lying in the same horizontal plane and perpendicular to the axis of rod 3I. The longest rod 32 of these four rods is integral with sleeve 30 and represents the diameter of a semicircle, said diameter being on the order of ten inches, for example; the other rods 33-35 are integral with rod 3| and have lengths corresponding to chords of said semi-circle; the axis of rod 3I is colinear with a radius of said semicircle. The axis of rod 3|, as well as the axes of rods 3235, is preferably spaced a distance on the order of a quarter to a half wavelength from the top wall of the cavity I. The axes of rods 3I35 lie in a common horizontal plane. The above-described rods 3I35 constitute the upper half or upper grid of stirrer structure 29. The lower half or lower grid of said stirrer structure is exactly like the upper half previously described, and consists of support rod 36 and four chordal rods 3l-40 whose axes are perpendicular to the axis of rod 36, the axes of all the'rods 6-48 being horizontal but the axes of rods ill-39 being perpendicular to the axis of rod 36. The only difference between the upper and lower halves of structure 29 is that the lower half 36-40 is displaced in azimuth with respect to the upper half, so that the axes of rods 3| and 36'lie at right angles to each other, and the axes of rods 3 -49 lie at right angles to the axes of rods 32-35.

The axes of the rods 36-40 of the lower half of the stirrer structure 29 lie in a common horizontal plane, this plane being spaced approximately a quarter-wavelength below the horizontal plane in which the axes-of rods 3I35 lie.

' The exciting rod of guide 42 is arranged'to set up TEo,1 mode waves therein, as will be apparent to those skilled in the art. These waves propagate down the waveguide and emanate from the open end thereof into the cavity I. It has been found that a large oven of the above-described type, containing therein such a rotating grid-like stirrer structure and excited by microwave energy from source in the above-described manner, produces a very uniform overall or integrated radio-frequency heat pattern throughout the entire interior of enclosure I, due to its stirring action. It is not known exactly how or why this result is accomplished, but our present understanding of the theory of operation of this modification is substantially as follows.

Cavity as in the previous embodiment, has linear dimensions which are large compared to the wavelength of the radio-frequency energy, so that a complex-mode radio-frequency field pattern is established in the enclosure or cavity, said pattern filling the entire volume of the enclosure in a more or less uniform manner, said cavity utilizing the input power thereto very eificiently because of the high Q thereof.

Considering the stirrer structure 29 in the particular momentary position of Fig. 4, in the first quadrant (algebraically speaking) there are no conducting rods whatever, so that in this area both horizontal polarization components of the radio-frequency waves pass beyond the stirrer structure to the top wall of the oven and are reflected therefrom. In the second quadrant there are the conducting rods 3235 extending from side to side of the oven parallel to dimension w, which rods act as a polarizer, permitting only the horizontal polarization component of the waves which is perpendicular to said rods (or parallel to dimension d) to pass through the stirrer structure in this area and reflecting from the stirrer structure the horizontal polarization component which is parallel to said rods (or parallel to dimension w). In the third quadrant, there are the conducting rods 3l4ll extending from front to rear of the oven parallel to dimension d and also the conducting rods 32-35 extending parallel to dimension w, the two sets of rods being spaced from each other. This structure causes both horizontal polarization components (that is,

the one parallel to dimension w and the one parallel to dimension d) to be reflected from the stirrer structure in thi quadrant, permitting neither of these components to pass therethrough. Rods -3235 reflect the component parallel to dimension w, while rods 31-40 reflect the component parallel to dimension d. In the fourth quadrant, there are the conducting rods 31-40 extending parallel to dimension d, which act to permit only the horizontal polarization component of the waves which is perpendicular to said rods (or parallel to dimension w) to pass through the stirrer structure in this area and to reflect from the stirrer structure the horizontal polarization component which is parallel to said rods (or parallel to dimension d).

. From the above discussion, we see that there is a different polarization-reflection characteristic in each of the four quadrants of the stirrer structure; that is, in the first no waves are reflected by or from the stirrer structure, in the second the horizontal polarization component parallel to dimension w is reflected by the stirrer structure, in the third both horizontal polarization components are reflected by the stirrer strutcure, and in the fourth the horizontal polarization component parallel to dimension d is reflected by the stirrer structure. Of course, those components not reflected by or from the stirrer structure pass on therethrough and are reflected from the top wall of the oven. Therefore, a wave having both horizontal components of polarization will "be.

differentially reflected by the stirrer structure in each of the four quadrants.

As the stirrer structure is rotated by motor 20, these different polarization-reflection characteristics will of course be varied or interchanged accordingly. Therefore, there will be continuouslyvarying polarization-reflection characteristics in each of the four quadrants, resulting'in a continuous effective changing of the wave modes present in the enclosure, or a changing of the field distribution which is the stirring action. The integrated heat is thereby made substantially uniform throughout the cavity. This can be appreciated from the fact that waves, which travel entirely through the stirrer structure before reflection, travel distances which are different from those travelled by the waves which are reflected from and by the stirrer structure.

Of course, it is to be understood that this invention is not restricted to the particular details as described above, as many equivalents will suggest themselves to those skilled in the art. For example, each of the grids of the grid-like stirrer structure 29 could cover a complete circle instead of a semi-circle, without departing from the spirit and scope of the invention. In this case, the same polarization-reflection characteristics would be provided in all four quadrants, but by rotation of the stirrer structure these characteristics would be intermittently applicable to the four quadrants, thus giving an effective stirring action. Also, other types of stirring devices could be rotated or reciprocated within the oven to produce the desired effect. In addition, other types of mechanical driving means could be utilized for the stirring devices disclosed. Various other variations will suggest themselves. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of this invention within the art.

What is claimed is:

1. Heating apparatus, comprising means for generating high-frequency electromagnetic energy, a high-Q metallic cavity, and means for coupling the energy output of said generating means to said cavity, said cavity includin means for enabling a body to be heated to be placed inside said cavity, and continuously movable radiating means for varying the mode pattern in said cavity.

2. High-frequency apparatus, comprising a metallic enclosure, means for supplying to said enclosure high-frequency electromagnetic waves having a predetermined wavelength, the internal dimensions of said enclosure being large compared to said wavelength, said waves setting up in said enclosure a predetermined electric field distribution pattern, movable means positioned in said enclosure for producing changes in said pattern, and means for moving said movable means in a continuous manner, said enclosure including means for enabling a body to be heated to be placed inside said enclosure. 7

3. High-frequency apparatus, comprising a metallic enclosure, means for supplying to said enclosure high-frequency electromagnetic waves having a predetermined wavelength, the internal dimensions of said enclosure being large compared to said wavelength, a movable metallic structure positioned in said enclosure, and means for moving said structure in a continuous manner, said enclosure including means for enabling a body to be heated to be placed inside said enclosure.

4. Heating apparatus, comprising means for generating high frequency electromagnetic energy, a high-Q metallic cavity, and energy radiating means for continuously variably distributing said energy throughout said cavity, said cavity including means for enabling a body to be heated to be placed inside said cavity.

5. Heating apparatus, comprising a metallic enclosure, means for generating microwave energy having a predetermined Wavelength, the internal dimensions of said enclosure being large compared to said wavelength, and energy radiating means for continuously variably distributing said energy substantially uniformly throughout said enclosure, said enclosure including means for enabling a body to be heated to be placed inside said enclosure.

6. Heating apparatus, comprising a metallic enclosure, means for supplying microwaves to said enclosure to set up therein a predetermined electric field distribution pattern, and energy radiating means associated With said enclosure for continuously producing changes in said pattern, said enclosure including means for enabling a body to be heated to be placed inside said enclosure.

7. Heating apparatus, comprising a metallic enclosure, means for generating microwave energy having a predetermined wavelength, the internal dimensions of said enclosure being large compared to said Wavelength, means for coupling the energy output of said generating means to said enclosure to set up therein a predetermined electric field distribution pattern, and energy radiating means associated With said enclosure for continuously producing periodic changes in said pattern, said enclosure including means for enabling a body to be heated to be placed inside said enclosure.

Heating apparatus, comprising an enclosure defined by substantially entirely closed energy reflecting surfaces, means for supplying high frequency electromagnetic waves to said enclosure, a movable metallic structure positioned in said enclosure, and means for moving said structure in a continuous manner, said enclosure including means for enabling a body to be heated to be placed inside said enclosure.

WILLIAM lri. HALL.

CITED Tile following references are of record in the tide of this patent:

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